This invention relates to a digital switching system and, more particularly, to a digital switching system with host and remote duplicated transmission controllers.
Since faults in switching systems cause serious inconveniences in a social file, system component devices cannot be suspended under any circumstances. It is, therefore, necessary to construct the system with both active devices and standby devices or in "ACT/STBY" structure to allow redundancy. Transmission lines should also be made redundant in order to maintain reliability if they connect switching offices over a long distance and in a severe environment.
An example of measures to meet such requirements in the digital switching system is disclosed in the "NEAX 61 Digital Remote Switching System" (NEC RESEARCH & DEVELOPMENT No. 64, pp. 86-93, published in January, 1982 by NEC Corporation. In the switching system described, host digital trunk interface controllers (HDTIC's) at a host office and remote local controllers (RLOC's) at a remote office are duplicated respectively by "ACT/STBY" redundancy structure. A plurality of digital transmission lines of a redundancy structure are provided between the duplicated HDTIC's (HDTIC.sub.0 and HDTIC.sub.1) and the duplicated RLOC's (RLOC.sub.0 and RLOC.sub.1) The HDTIC's and the RLOC's communicate with each other generally through one transmission line, but if a fault occurs in the transmission line, another transmission line must be selected and used for the communication. By utilizing this type of structure, even if faults occur in transmission lines, suspension of communication service can be avoided. Since only the HDTIC.sub.0 and the RLOC.sub.0 which are currently in active mode among the duplicated HDTIC's and RLOC's have the control over the transmission lines, or the right to select one of the transmission lines, and to transmit data, the HDTIC.sub.1 and the RLOC.sub.1 which are currently in standby mode are operated by monitoring the control information transmitted via the selected transmission line. When a fault occurs in the switching system which requires switching between active and standly modes, the HDTIC.sub.1 and the RLOC.sub.1 which have been in standby mode are often affected by the HDTIC.sub.0 and the RLOC.sub.0 to thereby become unable to continue smooth communication service. Further, as the HDTIC.sub.1 and the RLOC.sub.1 are in inactive statc, and when they are to be switched to active state, they must copy the operative state on the HDTIC.sub.0 and the RLOC.sub.0 by mate communication, communication service in such a separate mode is unavoidably suspended while the state is being copied.